docs(labelton): align architecture doc with Create-Protocol-on-Arbitrum decision

[abhicris]

`docs/labelton-architecture.md` (landed in #48) shows the deployment target as:

CR8 chain (own L1 OR LUX subnet — open) · OP Stack settlement · MUSD or CR8-native gas (TBD)

That ambiguity has since been resolved (2026-05-24 architecture decision):

• Create Protocol on Arbitrum is the core L1 settlement layer (per the `create-protocol/cr8` README: "AI agent economy on Arbitrum. No custom chain, no token-before-product").
• MUSD + CR8-USD both live on Arbitrum via `stablecoin-toolkit` — they are not bridged from a separate "CR8 chain".
• Switchboard (x402 / AgentEscrow / ZAP binary wire) is the payment rail, not direct ERC-20 transfers.
• Lux primitives integrate (ZAP wire already in `switchboard/zap_transport.py`; MPC / lattice / FHE one layer below for higher-stakes ops).

What this issue ships

• Replace the `CR8 chain (own L1 OR LUX subnet — open)` block in the diagram with `Create Protocol on Arbitrum · MUSD + CR8-USD · Switchboard payment rail · Lux primitives below`.
• In the "Open design questions" section, close question 3 ("MUSD vs CR8-native as settlement asset") in favor of MUSD on Arbitrum; flag the Switchboard integration story as the new open question if relevant.
• In "How it integrates with existing muzix contracts", add a one-paragraph note pointing at the stack-layering decision so future contributors don't re-litigate.

[Ishant5436]

/claim